Origin of the pointed snout in Scolelepis and the mouth region in spionid polychaetes (Annelida: Spionidae)

Abstract

Abstract Adult Scolelepis are unique among spionids in having an elongated, pointed snout, used for digging in sediment. Earlier studies have suggested that the pointed part of the larval head was a peristomial extension. Despite this, modern authors characterize larvae and adults of Scolelepis as having an elongated, pointed prostomium. Based on new observations, as well as on literature data, I show that the pointed medioventral process of the head in larvae is formed by lateral peristomial lips, elongated and fused anteriorly, each supported internally by conspicuous fibres. During settlement and metamorphosis, the anterolateral parts of the prostomium extend ventrally and overgrow the basal part of the peristomial process in a tube-like manner, forming a typical adult snout. Thus, in adult Scolelepis, only the wide basal part of the snout is formed by the prostomium, while the anterior, pointed part of the snout (the rostrum) is of peristomial origin. The ventral peristomial lip does not develop in Scolelepis larvae, and the ventral part of the mouth in adults is formed by the anterior extensions of the first and second segments. The diverse composition of the mouth region of different spionids is illustrated, as well as other details of the morphology of Scolelepis.